1. Field of the Invention
The present invention relates to a data communication system for exchanging data among a plurality of devices, a data communication device therefor, and a memory medium on which a data communication program is recorded.
2. Related Background Art
Conventionally, a communication system is well known wherein a whole processing device for controlling the entire system is connected by a cable with a device to be controlled, each device having an interface by which data can be exchanged and power can be supplied. A PBX or a public network, for example, has a function whereby communication, which is the basic function, is enabled during a blackout. Also, along an ISDN line, which is a digital public network, a maximum power of 420 mW (40 V) is separately supplied.
There are other systems that employ cables constituted by power feed lines and signal lines across which power is supplied and signals are exchanged. An example of such a system is one where a personal computer is connected by cables to a keyboard and a mouse. The implementation of this system is required because a keyboard and a mouse are absolutely necessary devices for a personal computer, and because there is a need to reduce the size and the weight of the devices and to eliminate the complicated wiring that is employed for accessories, such as the AC cord and a power adaptor.
Since the above described devices require only small amounts of power, sufficient power for their needs can be supplied by a station switch, a PBX or a PC (personal computer). Power supplied in this manner is not adequate for devices that require intermediate or large supplies of power, such as facsimile machines, multi-function telephones, monitors or printers, and external power sources are prepared for such devices to ensure a power capacity.
In particular, monitors and printers do not receive power from PCs, and are connected to the PC only by signal lines for data exchange and the transmission of control commands and status information.
However, in the above described conventional data communication system, when a device that is operated by power supplied by an external power source is powered off, the device is naturally halted. And when, for example, a PC is to examine the state of one of its peripheral devices, the device must be powered on temporarily and set to an operational state to enable the acquisition of necessary information. If the device is in the OFF state, the PC can not obtain information as to whether the device is powered off, no cable is connected to the device, or the device is in the operation disabled state.
In addition, a lack of paper, ink or toner, or a paper jam can not be detected unless a printer is powered on.
To resolve the above shortcomings, it is one object of the present invention to provide a data communication system that can detect the state of a device without powering the device on, and that can thus provide improved usability.